Nitrogen oxides contribute to the formation of ground-level ozone and, like sulfur dioxide, can cause respiratory system problems, according to the EPA.

Platte River also installed equipment in Rawhide to reduce mercury emissions two years before the state of Colorado required the reduction. The utility plans to reduce mercury emissions even further by 2018.

Other coal-fired plants nationwide have struggled to keep up with tightening air pollution regulations. Companies such as GenOn Energy Inc. and Midwest Generation separately announced closures of several coal-fired power plants last month.

The situation may only become more difficult for coal-fired plants.

In late March, EPA proposed the first federal standard to limit carbon pollution from new power plants. The move reflected a trend in the industry to build cleaner power plants, including ones using natural gas, the agency said.

The rule would not affect power plants like Rawhide or plants whose construction will start within the next year, according to EPA.

Bleem credits Rawhide’s relative youth – it started generating commercial power in 1984 – compared with older plants that lack the space to add new emission-control equipment. The power plant also pays more for low-sulfur coal from Wyoming’s Powder River Basin.

“We’re hoping to stay ahead of what the requirements are for future regulations,” Bleem said.

Rawhide maintains a 280-megawatt coal-fired unit as well as five natural gas-fired combustion turbines. Platte River, a wholesale provider of electricity to Fort Collins, Loveland, Longmont and Estes Park, operates the power plant.

Platte River first upgraded the plant to reduce nitrogen oxides by about 40 percent several years ago, Bleem said. The utility spent about $25 million on those upgrades at Rawhide and another coal-fired plant it operates in Craig.

Since then, Platte River has made additional changes to improve efficiency, which has resulted in lower emissions, Bleem said.

Kevin Cross, convener for the Fort Collins Sustainability Group, welcomed the emission reductions, but said he would like to see the plant generate a greater share of its electricity from natural gas and renewable energy.

Rawhide should be converted to burn natural gas to reduce its carbon dioxide emissions, Cross wrote in an email.

In addition, Platte River “should aggressively develop wind and solar generating capacity in order to reduce the use of all fossil fuels to a bare minimum amount within the next two decades,” Cross wrote.

Cities also should step up their efforts to reduce electric energy use by their customers, he added.

Platte River already generates electricity from renewable sources such wind energy and hydropower.